Powder coating compositions and techniques have been used in the provision of protective films for a considerable period of time. There is a substantial amount of prior art describing the use of blocked isocyanates in combination with hydroxylated polyesters for use in the preparation of powder coatings, adhesives and high solids, solvent based, fluid coatings. There have been several problems associated with heretofore known powder-coatings based on the use of blocked isocyanate curing agents. At present, commercially available blocked aliphatic isocyanates require at least about 160.degree. C. curing temperature for at least about 60 minutes to achieve a cure.
U.S. Pat No. 4,055,551 describes a continuous method for manufacture of blocked isocyanate curing agents to alleviate the problem of the formation of gel particles when batch reactions are used. The method described in this patent includes the steps of separately and continuously metering a polyisocyanate and a blocking agent for the polyisocyanate into a reaction zone in stoichiometric proportion under reaction conditions and in which the reaction mixture is continuously withdrawn from the reaction zone to provide a blocked isocyanate curing agent. The isocyanates particularly disclosed in this patent include isophorone diisocyanate, toluene diisocyanate, isocyanate polyol pre-polymers and mixtures thereof. The particular blocking agents used in this patent are epsilon caprolactam and methylethyl ketoxime.
In an article by P. McBride, "Lower Temperature Curing Blocked Isocyanate for Use in Powder Coatings," J. Oil Col. Chem. Assoc., 1982, 65, 257-262, the general requirements for formulating powder-coatings are discussed. As indicated in the McBride article, the technique of powder coating is well-known. In this method, the powder is electrically charged during the spray application and the substrate to be coated is grounded. Thus, the charged powder is attracted to the grounded substrate and covers the substrate. The coated substrate is then heated to a temperature above the melting point of the powder, the powder melts, flows out and a smooth, continuous finish results.
The pollution advantages of powder coating are that no solvent is involved and loss of raw material via venting to the atmosphere is minimized. However, the principal disadvantage of powder coatings, based on hydroxylated polyester resins, is that higher heating temperatures are required to cure the powder coatings than those required for liquid coatings. Such higher heating temperatures are required to dissociate the blocked isocyanates which are present to serve as a cross-linking agent for the hydroxylated polyester. A temperature of at least 180.degree. C. for fifteen minutes for powder coatings has often been required compared to 135.degree. C. for twenty-five minutes for liquid coatings. Thus, while powder coatings minimize the pollution problem, there is still a problem from an energy standpoint for a polyester-based powder coating and there is a need for a polyester-based powder coating that can be unblocked and cured at a lower temperature.
It is also pointed out in the McBride article that there are a number of conflicting requirements for powder coatings which actually limit the extent to which the cure temperature can be reduced. For example, the finely divided powder must remain in a free-flowing state even after extended storage. Agglomeration of the powder would render the powder unusable. Agglomeration is usually the result of cold flow which is related to the transition temperature (Tg). It is desirable to have a Tg of about 40.degree. C. or higher.
The parameters for an improved curing reaction for hydroxyl-functional polyesters set forth in the McBride article include: (1) effect the cure at temperatures of 140.degree.-160.degree. C. in less than 30 minutes; (2) emit no volatiles during curing; (3) impart good, physical properties to the finished coating; (4) permit all coating ingredients to be mixed without any pre-curing of the components occurring; (5) allow sufficient flow of the molten coating during fusion to give a high-gloss finish with good appearance; (6) allow stable storage of the powder coating.
The composition proposed in the McBride article to meet the above parameters for a hydroxyl-functional polyester based coating was an adduct of 1, 4-cyclohexane bis(methyl isocyanate) and diisobutyl ketoxime. The proposed composition was used as a curing agent for use with hydroxylated polyesters. The powder coatings provided by the adduct of 1,4-cyclohexane bis(methyl isocyanate) and diisobutyl ketoxime had a curing temperature of about 160.degree. C. While this composition is an improvement over the prior art which required curing temperatures of 180.degree. C. , it would be desirable to produce a blocked isocyanate which can be used to formulate hydroxylated polyester powder coating compositions which can be cured at a temperature of less than 150.degree. C., preferably at about 140.degree. C. or below. Powder coatings of the present invention can be unblocked and cured at temperatures of about 140.degree. C. or below.
Powder coating compositions usually involve a cross-linking reaction between a hydroxylated polyester and a diisocyanate. This is an addition reaction involving no elimination products. A significant problem, however, is that the reaction can take place at temperatures as low as 90.degree. C. in thirty minutes. Therefore, it has been necessary to "block" the isocyanate groups in a manner which render them inactive at temperatures below 180.degree. C. but allow the regeneration of the isocyanate groups above this temperature. Isocyanates are usually blocked by reaction with a number of active hydrogen compounds, such as lactams or ketoximes. As indicated above, it is desirable to provide a blocked isocyanate component of a powder coating composition which has an unblocking or disassociation temperature lower than 180.degree. C. and preferably lower than 150.degree. C. Most preferably, it would be desirable to provide an isocyanate component that could be unblocked at temperatures of about 140.degree. C. Thus far, the prior art has not provided a blocked isocyanate for use in conjunction with hydroxylated polyesters which can be unblocked at temperatures of about 140.degree. C. This is in spite of the fact that many potential isocyanate materials and blocking agents have been proposed. For example, in U.S. Pat. No. 3,931,117 to Leonard, the following isocyanate materials were proposed for use in a coating powder: isophorone diisocyanate, 4,4'-methylene bis-(cycohexyl isocyanate), toluene diisocyanate, tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, ethylene diisocyanate, cyclopentylene-1,3-diisocyanate, cyclohexylene-1,3-diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-napthalene diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, toluene trisisocyanate, 1,4-xylene diisocyanate, and 4,4'-diphenylmethane diisocyanate.
Among the blocking agents proposed for use in combination with the isocyanate, epsilon caprolactam, methylethyl ketoxime and acetone oxime have been proposed. The McBride reference specifically proposes the use of diisobutyl ketoxime in combination with 1,4-cyclohexane bis(methyl isocyanate). U.S. Pat. No. 4,055,551 to Panandiker et al. proposes the use of epsilon caprolactam or methylethyl ketoxime. U.S. Pat. No. 4,649,067 to Gras proposes the use of ethyl caprolactam or acetoxime.
It has also been proposed, such as described in U.S. Pat. No. 4,055,051 to Panandiker that, a polyisocyanate can be extended to provide a pre-polymer with urethane linkages by reacting the polyisocyanate with a polyol. Various polyols have been suggested for use, such as trimethylol propane, neopentyl glycol, cyclohexane dimethanol, alkylene glycols and other aliphatics diols and triols, such as pentaerithritol, ethylene glycol, hydroxy-polycaprolactones, trimethylol ethane, tris(hydroxy ethyl) isocyanurate and mixtures of these compounds.
Accordingly, it is a principal object of the present invention to provide a particular blocked isocyanate composition which can be used as a cross-linking agent for hydroxylated polyesters.
It is another object of the present invention to provide a blocked isocyanate composition from the reaction product of tetramethylxylene diisocyanate and acetone oxime for use as curing agent for powder coatings, adhesives and high solids baking coatings.
It is a further object of the present invention to provide a powder coating composition which can be cured at temperatures as low as 130.degree. C. in periods of time of as low as thirty minutes.
It is a still further object of the present invention to provide a blocked isocyanate composition from tetramethylxylene diisocyanate which has been extended with a polyhydric compound and which has been blocked by use of acetone oxime.
These and other objects will become more apparent from the following description and the appended claims.